Recently, importance has been attached to solve energy and environmental problems. Therefore, it is demanded to provide an electric power source of high energy density, the emission matter of which is clean. The fuel cell is a generator, the energy density of which is several times as high as that of the conventional battery. The energy efficiency of the fuel cell is high. Further, the exhaust gas discharged from the fuel cell contains no nitrogen oxide and sulfur oxide. Alternatively, the exhaust gas discharged from the fuel cell seldom contains nitrogen oxide and sulfur oxide. Accordingly, it is said that the fuel cell is a very effective device which meets the demand for an electric power source device of the next generation. Especially, the solid polymer type fuel cell has a good starting characteristic because it can be driven even at low temperatures of not higher than 100° C. Therefore, the solid polymer type fuel cell has been actively developed so that it can be used as a stationary dispersion type power source, an automobile power source and a power source incorporated into a portable device.
The high molecular type fuel cell is a device operated in such a manner that the oxidization of hydrogen on the anode and the reduction of oxygen on the cathode are electrochemically conducted at the same time, and an electrical output can be obtained by an electric current taken out from a potential difference between the anode and cathode in the electrochemical reaction process. The conventional fuel cell includes: a fuel storage section for storing a reactant; a reacting section for reforming the reactant to fuel gas; a fuel supply passage; an anode or cathode for generating electricity by the electrochemical reaction of fuel; and an electrolyte for transmitting ions between the anode and cathode.
Examples of the anode side fuel conventionally used are: hydrogen, alcohol such as methanol or ethanol; ether; and chemical hydride such as cyclohexanol or sodium boron hydride. Except for hydrogen, all the fuel described above is used in the form of liquid and transformed into hydrogen gas by are forming device. Concerning the fuel cell, the developer's attention has been focused upon the selection of chemical substance which is effective for taking out hydrogen and suitable for transporting and storing the fuel. Attention is given to the fuel described above because the fuel is assumed to be effective for the fuel cell.
The reactant on the cathode side is an oxidant. The typical oxidant is oxygen. However, a peroxide such as hydrogen peroxide is used in some cases.
In order to drive a device, which consumes electric power, such as an electrical appliance, a portable device and an automobile by a fuel cell, it is necessary for the fuel cell to output electric power, the intensity of which corresponds to a load of the device which consumes electric power. An output of the fuel cell is a factor determined by a volume of the reacting gas to be supplied to the electrode such as hydrogen gas or oxygen gas. Accordingly, when an extremely large volume of the reacting gas exists in the periphery of the electrode, an output of the fuel cell can be changed responding to the load given to the fuel cell. From this viewpoint, when the reacting gas is appropriately supplied to the electrode before hydrogen and oxygen existing in the periphery of the electrode are used up, it is possible for the fuel cell to be operated responding to the load of the device which consumes electric power.
In this case, in order to supply a necessary volume of the reacting gas, it is necessary to obtain the necessary volume of the reacting gas from the above fuel and oxidant. Accordingly, an appropriate volume of fuel must be supplied from the fuel storage section to the reacting section. Further, an appropriate volume of oxidant must be supplied to the reacting section.
When the fuel, oxidant and reacting gas are supplied, it is conventional to use a pump or blower. The volumes to be supplied are controlled according to the output of the fuel cell and the load of the device which consumes electric power. Concerning this matter, for example, refer to Non-patent Document 1.
Non-patent Document 1: “Technical Tendency of Portable Type Fuel Cell” by Masahiro Ichimura, pages 2 to 4 and FIG. 3 in NTT Building Technology Institute 2003
However, in any of the anode and the cathode, it is difficult to control the volumes of fuel and reacting gas to be supplied to the electrode. In order to control the volumes of fuel and reacting gas, it is necessary to input energy, which is used for controlling, from the outside or the fuel cell itself into the control system. Accordingly, since energy is consumed for controlling as described above, the effective output of the fuel cell is decreased.
Further, in the case where hydrogen is used as the fuel on the anode side, when hydrogen is taken out by reforming the reactant, it is necessary to control a volume of the generated hydrogen according to a volume of hydrogen used on the anode. In order to control the volume of the generated hydrogen, it is necessary to control a reaction temperature and a volume of fuel to be supplied. Therefore, it is necessary to attach a temperature control system such as a heater, temperature sensor and controller to the reacting section and the electrode. Further, it is necessary to attach a valve and controller for controlling the volume of fuel to be supplied. Accordingly, energy is further consumed by the above control mechanism for controlling the reaction. Therefore, the effective output of the fuel cell is decreased.
At the same time, especially, in the case of a fuel cell applied to a small electronic device, a space, in which fuel is to be accommodated, is decreased by the volume in which the above system is arranged. Accordingly, the volume of this system is very disadvantageous in the volume energy density. By the volume of this system, the volume energy density of this fuel cell is lowered as compared with the volume energy density of the conventional battery.
In the case where the system of controlling a volume of the generated hydrogen is not provided, the inner pressure in the fuel cell is raised by the hydrogen generated exceeding a volume of hydrogen corresponding to an electric current to be outputted. In this case, the generated hydrogen cross-leaks to the cathode side through the solid high molecular electrolyte film, and an output of the cathode is lowered.
When the hydrogen leaks out as described above, it is impossible to effectively use the hydrogen. Therefore, the energy density is lowered.
Problems caused on the anode are described above, however, the same problems are also caused on the cathode. In the case where oxygen is generated from the oxidant and the thus generated oxygen is supplied to the cathode, it is necessary to control the volume of the generated oxygen according to the volume of the used oxygen. Energy is consumed for this control. Further, in order to arrange the controlling mechanism, the volume energy density is decreased.
It is an object of the present invention to decrease energy necessary for supplying appropriate volumes of fuel and oxidant to an electrode of a fuel cell so as to decrease a volume of the control mechanism for controlling a volume of the supply and also decrease a volume of the control mechanism for controlling a volume of the reaction. It is another object of the present invention to solve the above problems in the conventional art by providing a small fuel cell system, which is used for a portable device, characterized in that the energy density is advantageously high, the size of the fuel cell is small, the fuel cell can be safely operated and the fuel utilizing efficiency is high.